Based on difference in light emitting materials, white organic light-emitting diodes (OLED) can be divided into full-fluorescent white light device, full-phosphorescent white light device, and fluorescent/phosphorescent mixed white light device. In the full-fluorescent white light device, only singlet excitons which account for 25% of total excitons can be utilized by a fluorescent dye, while remaining triplet excitons which account for 75% of the total excitons cannot be effectively utilized. The triplet excitons are dissipated in a form of heat radiation finally, and cannot make a contribution to light emission. Therefore, a theoretical maximum value of internal quantum efficiency of the full-fluorescent white light device is only 25%. As to the full-phosphorescent white light device, by an action of heavy atom coupling, energy levels of singlet excitons and triplet excitons are mutually mixed, so that energy of the triplet excitons, which is originally prohibited, is relieved and phosphorescence is emitted. Therefore, internal quantum efficiency of the full-phosphorescent white light device can reach 100% theoretically. At present, white light devices having high efficiency which are reported in the literature are all white light devices based on full-phosphorescent. However, due to a limitation caused by unsatisfactory luminescence lifetime of a blue phosphorescent dye, the full-phosphorescent white light device cannot satisfy actual application requirements in most fields. As to the fluorescent/phosphorescent mixed white light device, a stable blue fluorescent dye and a long-wavelength phosphorescent dye with high efficiency are used together. These two luminescent dyes work together, so that stability can be ensured and high efficiency can be obtained at the same time for the fluorescent/phosphorescent mixed white light device. Therefore, difficulties for the full-fluorescent white light device and the full-phosphorescent white light device are solved effectively. The fluorescent/phosphorescent mixed white light device becomes a better choice, and it has attracted wide attentions in the industry.
At present, as to the fluorescent/phosphorescent mixed white light devices, there are still some urgent problems which need to be solved. (1) Structural styles are too monotonous, and there is a lack of novel structural designs. (2) Compared with a full-phosphorescent white light device having high efficiency, efficiency of a fluorescent/phosphorescent mixed white light device is still relatively low. (3) When a fluorescent/phosphorescent mixed white light device has high brightness, efficiency attenuation thereof is too fast. (4) High CRI (color rendering index), good CIE (coherent infrared energy) and stable spectra cannot be ensured at the same time.